A Preliminary Study On Larval Settlement And Metamorphosis Of The Sea Cucumber (Apostichopus Japonicus) In Response To Benthic Diatoms

The problem of low settlement and metamorphosis rate on sea cucumber larvalhas restricted the development of sea cucumber aquaculture,thereby increasing theefficiency of the sea cucumber seedlings is one of the key factors to promote thehealthy development of sea cucumber aquaculture. Benthic diatoms are rich innutrients, which can be used as a settlement and metamorphosis abalone on seacucumbers、 sea urchins and other valuable marine larval，and also are the necessaryfood of the larval after metamorphosis. Therefore, the quality and density of benthicdiatoms is one of the key success factors of benthic marine economic animal breeding,and also affects the growth and survivalrate.However，there are few reports on larvalsettlement and metamorphosis of the sea cucumber (Apostichopus japonicus) inresponse to benthic diatoms.In this study, we isolates many kinds of benthic diatoms with strong adhesionfrom the natural sea and sea cucumber aquaculture ponds,and then identificates thespecies and explores the attachment growth law ofbenthic diatom.The experimentattaches to the formation of different concentrations of benthic diatoms substrate，andanalysis the induced effects of a single species of benthic diatoms by the larvaesettlement and metamorphosis rate.It also explores the inducted effects that thebenthic diatoms were combined to benthic diatoms and active bacteria.Finally, itmakes sure the species of benthicdiatoms with high-inducing activity, and analysis thesynergistic effects by different combination style.Details on four main studies of thethesis are as follows:1. Separation and purification of benthic diatomsThrough methods such as single-cell separation, experiment gets a total of onehundred plants of the diatoms,which are preliminarily determined to be12different kinds by microscopic examination.Nine kinds of the benthic diatoms are elected tocarry on the induced experiment,and then gets the identification results of sevenkinds of benthic diatoms,which are Nitzschia sp.、 Bacillariophyta sp.1、Bacillariophyta sp.2、 Bacillariophyta sp.3、Bacillariophyta sp.4、Bacillariophytasp.5、Bacillariophyta sp.6.2. Effect of inducing sea cucumber larvae settlement and metamorphosis bynine kinds of benthic diatomsThe experiment induces sea cucumber larvae settlementand metamorphosis bynine kinds of benthic diatoms, the result shows that:72hours later，there are fourkinds of benthic diatoms’(such as Nitzschia sp.) inducted effect are significantlybetter than the blank group, and two kinds of diatoms’(such as Bacillariophytasp.2)induced effect are not obvious, however，three kinds of benthic diatoms’(such asBacillariophyta sp.4)are less effective than the blank group. Among them, the bestinduced effect of benthic diatom is Nitzschia sp.，when the cell density respectivelyreached (3.40±0.60)×10~3、(9.41±1.36)×10~4、(2.25±0.06)×10~5cells/cm~2，thesettlement and metamorphosis rate is56.67±5.77%、85±12.91%and67.5±17.08%，significantly better than the blank group（13.33±5.77%）,(P <0.05).The experimentprompts that there is a certain appropriate range of the density of benthic diatom forlarval settlement and metamorphosis.3. Effectof inducing sea cucumber larvae settlement and metamorphosis bythemixing ofBenthic DiatomsAccording to the induced effect of single species benthic diatom,the experimentmades the stratums for mixedbenthic diatoms Nitzschia sp. with Bacillariophyta sp.1、Bacillariophyta sp.4with WH14， which are also used for settlement andmetamorphosis. The result shows that:72hours later，the induced effect of mixedbenthic diatom stratum varied by the species and density of benthic diatom. When thecell density of two kinds of benthic diatoms is (1.41±0.13)×10~5and (1.24±0.18)×10~5cells/cm~2，the induced rate is increased to86.67±5.77%from67.5±17.08%and 40.00±20.00%，showing non-synergistic induced effect；the combination experimentof the two none-induction benthic diatoms also shows significantly synergisticinduced effect (P＜0.05). The experiment prompts that the better indued effects is dueto the increasing density of benthic diatom in the stratum for low active benthicdiatoms mixed.4. Effectof inducing sea cucumber larvae settlement and metamorphosis by thecombination ofBenthic Diatoms and Active BacteriasAccording to the induced effect of single species benthic diatom,the experimentmades the stratums for mixed benthic diatoms Nitzschia sp. and Bacillariophyta sp.4with active bacteria Pseudoalteromonas sp，which are also used for settlement andmetamorphosis.The results shows that,72hours later，stratum-induced effect is alsosubject to different species and density of the of benthic diatoms and active bacteria.when the cell density of benthic diatoms is（2.71±0.49）×10~5cells/cm~2and the celldensity of active bacteria is (5.39±0.53)×10~6cells/cm~2, the settlement andmetamorphosis rate is increased to80.00±8.16%from10.00±0.00%and46.67±15.28%，showing a significantly synergistic induced effect（P＜0.05）.In summary, the experiments shows that：benthic diatoms has the induced effecton the sea cucumber larvae settlement and metamorphosis.If the active benthic diatomare used forthe substrate construct in seed production,it can effectively improve therate of larval settlement and metamorphosis，and thus provide an effective technicalapproach to the optimization of the sea cucumber seed production technology.